The next day, drain the cashews and add them to a high speed blender. Add in the 1/3 cup water, vinegar, and salt. Blend until smooth. Chill until ready to use.
In a large pot or Dutch Oven, heat the avocado oil over medium heat. Add in the diced onion and salt. Sauté until the onion is translucent, about 7-10 minutes.